Switching power supplies are commonly used to provide electric power to many electronic devices. A transformer or an inductor is usually used as a tank element in the switching power supply. For example, an inductor is used in a BUCK converter. A main switch is electrically coupled to the inductor. The switch is turned on and off so as to alternately store energy in the inductor and transfer the stored energy to the load.
There are various control methods used in the switching power supply. The most popular one is fixed frequency peak current control. In this control method, the output signal of the switching power supply is sensed and compared with a reference signal to generate a feedback signal. The current flowing through the main switch is also sensed and a current sensing signal is generated. The current sensing signal is compared with the feedback signal. The main switch is turned off once the current sensing signal becomes larger than the feedback signal and turned on at a fixed frequency.
As well known, in the fixed frequency peak current control, a slope compensation signal should be added to the current sensing signal to suppress sub-harmonic oscillation when the duty cycle of the main switch is greater than 50%. The slope compensation signal is designed to cover the worst condition where the output voltage is the largest and the inductor is the smallest, so the slope compensation signal usually takes significant headroom when the duty cycle is large.
Generally, the main switch has a minimum off time period which limits the duty cycle. When the switching power supply needs to operate at a very high duty cycle, such as in a low dropout BUCK converter, a common practice is to extend the on time while keeping the off time fixed, hence the switching power supply is actually running in a fixed off time mode. In the fixed off time mode, the main switch is turned off once the current sensing signal becomes larger than the feedback signal, and turned on when the off time of the main switch reaches the minimum off time period.
The fixed off time mode doesn't require the slope compensation signal for stability. Also since the compensation slope signal takes significant headroom, to prevent the control circuit from running out of the headroom when the on time is extended, the peak amplitude of the slope compensation is usually clamped to a predetermined level after the switching power supply enters the fixed off time mode.
However, after the slope compensation signal is clamped, the falling edge of the control signal may be very sensitive to the noise on either the feedback signal or the current sensing signal if the ramp of the current sensing signal is too small. For a BUCK converter, the ramp of the current sensing signal purely depends on the inductance and the difference between the input voltage and output voltage. In conditions where the input and output voltages are very close, the ramp of the current sensing signal becomes so small that the switching frequency will be unstable due to noise in the system.